Code Division Multiple Access (CDMA) protocol involves the use of a unique code to distinguish each user's data signal from other users' data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user's message at the receiving end of the communication channel. There are two principal types of CDMA protocols, classified by the specific technique that is used to spread the user's data over a wide portion of the frequency spectrum: direct sequence (or pseudo-noise) and frequency hopping systems. The technical foundations for CDMA protocols are discussed, for example, in the recent book by Prasad entitled "CDMA for Wireless Personal Communications", Artech House, 1996.
The Direct Sequence CDMA (DS-CDMA) protocol involves the spreading of a user's data signal over a wide portion of the frequency spectrum by modulating the data signal with a unique code signal that is of higher bandwidth than the data signal. The frequency of the code signal is chosen to be much larger than the frequency of the data signal. The data signal is directly modulated by the code signal and the resulting encoded data signal modulates a single, wideband carrier that continuously covers a wide frequency range. After transmission of the DS-CDMA modulated carrier signal, the receiver uses a locally generated version of the user's unique code signal to demodulate the received signal and obtain a reconstructed data signal. The receiver is thus able to extract the user's data signal from a modulated carrier that bears many other users' data signals.
The Frequency Hopping CDMA (FH-CDMA) protocol involves the use of a unique code to change the value of a narrowband carrier frequency for successive bursts of the user's data signal. The value of the carrier frequency varies in time over a wide range of the frequency spectrum in accordance with the unique code. CDMA protocols are closely related to spread spectrum technology and the term Spread Spectrum Multiple Access (SSMA) is also used for CDMA protocols such as DS-CDMA and FH-CDMA that use a relatively wide frequency range over which to distribute a relatively narrowband data signal.
Mobile stations operating with the CDMA protocol transmit under strict power control. One of the implications of spreading a message signal over a wide band is that each transmitted signal must received by the base station at similar power levels. Thus, mobile stations that are located farther away from the base station must transmit their signals at a much higher power level than mobile stations near a base station, so that both signals may be received at the base station at equivalent power levels. The base station and the mobile stations operating with the CDMA protocol use a power control algorithm consisting of open-loop and closed-loop power control methods, to control the transmission power at the mobile stations.
In open-loop power control, each mobile station measures the signal power of the down-link message it received from the base station. Based on this measurement, and based on a prescribed target value, the mobile station then computes how much to adjust its own transmission power to achieve the desired received power level at the base station.
In closed-loop power control, the base station receives each mobile station's signal and measures its received power level. The base station determines if each received power level matches a target value for received power level. Then, based on this determination, the base station periodically transmits a power control message to each mobile station, multiplexing it with the down-link data. The power control message indicates to the mobile if it should increase or decrease its transmitted power so as to maintain the desired received power level at the base station.
The final transmit power adjustment performed by the mobile station is based on the combination of the open-loop and the closed-loop power control methods. The mobile station computes how much it has to increase or decrease its transmit power level on the open loop-measurement. It then listens to the base station's power control message to determine the closed-loop adjustment The mobile station then adds the open-loop adjustment and the closed loop adjustment to compute its resultant transmit power adjustment.
Currently, battery power conservation for mobile terminals operating in a wireless communications system is a significant concern, especially for wireless service providers. This is due primarily to the significant amounts of power expended for wireless transmission by the mobile terminal transmitter. Revenue cannot be generated if mobile terminals cannot complete calls due to their battery power being exhausted. One manual solution is the use of auxiliary power adapters to provide the power source needed for operation of the mobile terminal. However, this solution burdens wireless subscribers with additional costs and requires them to maintain a close proximity of the mobile terminal to the power source, to guarantee wireless service. Passive solutions to the problem involve scheduling the mobile terminal for sleep mode operation. The mobile terminal is instructed by the servicing base station to "wake up" at predetermined intervals to receive RF transmissions from the base station and to transmit RF signals to the base station. Although this solution does provide some relief, it also leads to significant over-the-air delays in the communications link since the base station and mobile terminal must continually buffer messages. This delay is magnified further in a wireless communications system that operates on a reservation based protocol. Such a protocol directs mobile terminals to use a limited number of channels at specific times to reduce interference and collisions in the wireless link.
Due to the limitations of the prior art, there is a need for a solution that takes a more active, direct role in managing mobile terminals operating with low battery power levels.